My tach is reading about 3300 at idle, and goes proportionally higher as I run the engine up. Let me tell you what I've done.I was given an 88 body with no engine or tranny, it originallt had a 305 with TBI. I transplanted my 305 and 700R4 from my 83 into it. I pulled the computer and all ecm wiring out. I have an edlebrock performer intake, Holley Carburator, and a vacuum advance HEI distributor. I modified the tranny to run the overdrive without the computer. The engine runs great, and the tranny is fine. My problem is the tach is reading about 3300 when I am at idle (somewhere aroud 800 to 900). When i barely run the engine up, it climbs and goes red line. I know the car have a v-8 to begin with, and have the single white wire on the tach output of the distributor. On the engine I am using Bosch Platinum plugs. Someone mentioned I may be getting feedback through the plugs causing noise in the distributor which is showing up on the tach. I have had a chance to change the plugs yet to try it. I was just wondering if someone may have another idea to go with, or may have come across this before.
Thanks for any help,
Bobby

 

I had a similar problem when I removed the instrument cluster without disconnecting the battery. When I put everything back together and started the car, my tachometer went to about 4500 at idle even thought the idle was normal. I had to disconnect the battery and remove the faceplate and clear plastic panel from the instrument cluster and reset the tachometer to ZERO manually with my finger. It may take a couple of attempts to get the needle to stay at ZERO, but it did work. Re-connect the battery and start the car to check it out before you put the instrument cluster back together.